Method for controlling discharge of power storage device

ABSTRACT

A method for controlling a discharge of a power storage device includes: a first step of discharging, in a first section of a discharging period, a part of an electric charge stored in a power storage through a first discharge path and a second discharge path of a discharge circuit; and a second step of discharging, in a second section of the discharging period, a remaining part of the electric charge through the second discharge path. The first discharge path includes a zener diode connected to one end of the power storage. The second discharge path includes a first transistor connected to the one end of the power storage. The first section is a period in which a voltage of the one end of the power storage is higher than a breakdown voltage of the zener diode. The second section is a period in which a voltage of the one end of the power storage is lower than the breakdown voltage of the zener diode. An output current of the first transistor in the second section is greater than an output current of the first transistor in the first section.

CROSS-REFERENCE OF RELATED APPLICATIONS

This application is a Continuation of the U.S. patent application Ser. No. 16/114,581, filed on Aug. 18, 2018, which is a continuation of the PCT International Patent Application No. PCT/JP2017/001885 filed on Jan. 20, 2017, which claims the benefit of Japanese patent application No. 2016-054334 filed on Mar. 17, 2016, the entire disclosures of which applications are incorporated by reference herein.

TECHNICAL FIELD

The present disclosure relates to a method for controlling a discharge of an electric charge stored in a capacitor of a power storage device.

BACKGROUND

Heretofore, there has been known a discharge circuit for discharging power charged in a capacitor in a power conversion apparatus such as an inverter apparatus.

As an example of this type of discharge circuit, a constant power discharge circuit is disclosed (refer to Unexamined Japanese Patent Publication No. 2009-112156). The constant power discharge circuit includes: a transistor capable of adjusting a discharge current by a gate voltage; and a shunt resistor that allows a flow of the discharge current adjusted by the transistor and outputs a reference voltage. The constant power discharge circuit decides discharge current setting values in response to a discharge voltage that gradually decreases due to the discharge. Moreover, the constant power discharge circuit compares one of the discharge current setting values selected in response to a residual voltage of the capacitor and the discharge current obtained from the reference voltage with each other and controls the gate voltage such that the discharge current becomes equal to the discharge current setting value.

SUMMARY

A method for controlling a discharge of a power storage device, the power storage device including a power storage and a discharge circuit, the method includes: a first step of discharging, in a first section of a discharging period, a part of an electric charge stored in the power storage through a first discharge path and a second discharge path which are included in the discharge circuit; and a second step of discharging, in a second section of the discharging period, a remaining part of the electric charge stored in the power storage through the second discharge path. The first discharge path includes a zener diode connected to one end of the power storage. The second discharge path includes a first transistor connected to the one end of the power storage at a first input terminal of the first transistor. The first section is a period in which a voltage of the one end of the power storage is higher than a breakdown voltage of the zener diode. The second section is a period in which a voltage of the one end of the power storage is lower than the breakdown voltage of the zener diode. An output current of the first transistor in the second section is greater than an output current of the first transistor in the first section.

In accordance with the present disclosure, a control of the discharge of the power storage device that performs discharge approximating constant power discharge can be realized with a simple configuration. Accordingly, a power storage device capable of decreasing size and cost can be implemented.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a circuit diagram illustrating a discharge circuit and a power storage device according to an exemplary embodiment of the present disclosure.

FIG. 2 is a schematic graph showing diode characteristics of a zener diode provided in the discharge circuit.

FIG. 3 is a graph showing simulation results of operation states for respective constituents of the power storage device and the discharge circuit according to the exemplary embodiment of the present disclosure.

DESCRIPTION OF EMBODIMENT

In the discharge circuit disclosed in Unexamined Japanese Patent Publication No. 2009-112156, it is necessary to provide the discharge current setting value for each of predetermined sections on a waveform of the residual voltage of the capacitor, and further, to control the gate voltage such that the actual discharge current becomes equal to the discharge current setting value.

In this connection, the present disclosure provides a method that controls a discharge current of a capacitor with a simple configuration without providing the foregoing discharge current setting values.

Hereinafter, a discharge circuit and a power storage device according to an exemplary embodiment of the present disclosure will be described with reference to the drawings. The exemplary embodiment described below is a preferred specific example of the present disclosure. Numeric values, shapes, materials, constituents, dispositions and connection modes of the constituents, and the like, which are shown in the following exemplary embodiment, are merely examples, and are not intended to limit the present disclosure. Accordingly, among the constituents in the following exemplary embodiment, constituents which are not recited in the independent claim for the most generic concept of the present disclosure are described as arbitrary constituents.

The drawings are also schematic diagrams and are not always exactly illustrated. In the drawings, substantially the same constituents are denoted by the same reference numerals, and a redundant description is omitted or simplified as appropriate.

Moreover, in the following exemplary embodiment, “connected” means to be electrically connected, and includes not only the case of being directly connected but also being indirectly connected via another electric element or the like.

Exemplary Embodiment [Configuration of Power Storage Device and Discharge Circuit]

FIG. 1 is a circuit diagram illustrating discharge circuit 10 and power storage device 1 according to the exemplary embodiment of the present disclosure.

Power storage device 1 according to the present exemplary embodiment, for example, is described by taking power storage device 1 mounted on a vehicle such as an automobile as an example.

Power storage device 1 according to the present exemplary embodiment includes: battery BATT mounted on the vehicle; capacitor C₁ connected to battery BATT; discharge circuit 10 connected to capacitor C₁; and controller 20. Here, capacitor C₁ is an example of a power storage of the present disclosure.

Battery BATT is connected to one end (positive electrode) cp of capacitor C₁ via switch Q₄. For example, switch Q₄ is composed of a field effect transistor (FET). Capacitor C₁ is charged by turning on switch Q₄, and stops being charged by turning off switch Q₄. Moreover, the other end (negative electrode) of capacitor C₁ is grounded.

Discharge circuit 10 is used for discharging power charged in capacitor C₁. Discharge circuit 10 includes: transistor Q₁ for controlling the discharge current of capacitor C₁; operational amplifier IC₁ for controlling an output current of transistor Q₁; current mirror circuit 15 connected to non-inverting input terminal (V+) of operational amplifier IC; zener diode ZD₁ connected to an input side of current mirror circuit 15; and reference power supply REF connected to an output side of current mirror circuit 15. Note that transistor Q₁ is an example of a first transistor of the present disclosure. Moreover, reference power supply REF is an example of a current source of the present disclosure.

In the present exemplary embodiment, transistor Q₁ is an n-channel FET. A current input terminal (drain) of transistor Q₁ is connected to one end cp of capacitor C₁ via resistor R₁, and a current output terminal (source) of transistor Q₁ is grounded via resistor R₂. An output terminal of operational amplifier IC₁ is connected to a control terminal (gate) of transistor Q₁, and operational amplifier IC₁ controls a voltage to be applied to the gate of transistor Q₁. In this way, magnitude of drain current I_(D) flowing between two current terminals (between the drain and the source) is controlled. Note that the source of transistor Q₁ is an example of an output terminal of the present disclosure.

The output terminal of operational amplifier IC₁ is connected to inverting input terminal (V−) of operational amplifier IC₁ via resistors R₃ and R₄.

Current mirror circuit 15 is a circuit for flowing, through an output side, a current of the same value as that of a current flowing through an input side. Current mirror circuit 15 includes transistors Q₂ and Q₃. Bases of transistors Q₂ and Q₃ are connected to each other, and the base and collector of transistor Q₃ are also connected to each other. Here, when transistor Q₂ is on the output side, transistor Q₃ is on the input side, and current I₁ flows between the collector and emitter of transistor Q₃, then current I₂ of substantially the same magnitude also flows between a collector and emitter of transistor Q₂ (I_(I)≈I₂). Note that transistors Q₂ and Q₃ are examples of second and third transistors of the present disclosure.

Reference power supply REF for outputting a constant voltage is connected to the collector of transistor Q₂ via resistors R₅ and R_(b). The emitter of transistor Q₂ is grounded via resistor R₇. Moreover, non-inverting input terminal (V+) of operational amplifier IC₁ is connected to a node ER between resistor R₅ and resistor R₆.

An anode of zener diode ZD₁ is connected to the collector of transistor Q₃ via resistor R₈, and a cathode of zener diode ZD₁ is connected to one end cp of capacitor C₁. The emitter of transistor Q₃ is grounded via resistor R₉. Note that resistors R₈ and R₉ are resistors for limiting a current flowing through zener diode ZD₁.

FIG. 2 is a schematic graph showing diode characteristics of zener diode ZD₁ provided in discharge circuit 10. As illustrated in FIG. 2, when a voltage of capacitor C₁ is larger than breakdown voltage V_(Z) of zener diode ZD₁ (first section), current I₁ flows through zener diode ZD₁ with ease. I₁ is maintained at a constant value until the voltage of capacitor C₁ reaches V_(Z) due to a general feature of the zener diode. Moreover, when the voltage of capacitor C₁ is smaller than breakdown voltage V_(Z) (second section), current I₁ flowing through zener diode ZD₁ decreases exponentially.

An output terminal of controller 20 is connected to inverting input terminal (V−) of operational amplifier IC₁ via resistor R₁₁ and diode D₁. One of input terminals of controller 20 is connected to the source of transistor Q₁ via resistor R₁₀, and the other of the input terminals of controller 20 is connected to one end cp of capacitor C₁. In this way, controller 20 can measure source voltage ES of transistor Q₁ and the voltage of capacitor C₁. Resistor R₁₀ is a protective resistor for protecting controller 20.

As described above, power storage device 1 of the present exemplary embodiment is configured.

[Operations of Power Storage Device and Discharge Circuit]

Next, a description will be given of operations of power storage device 1 and discharge circuit 10 according to the exemplary embodiment of the present disclosure.

Battery BATT is used for supplying power to an engine starter and an in-vehicle electrical instrument and for charging capacitor C₁. Capacitor C₁ is used in place of (or for backup of) battery BATT. Accordingly, when battery BATT is capable of supplying power to the in-vehicle instrument, for example, when an ignition key of the vehicle is turned on, then capacitor C₁ is fully charged by supplying power to battery BATT. However, when capacitor C₁ continues to be fully charged for a long time, then capacitor C₁ receives a stress and is prone to deteriorate. Accordingly, it is preferable that capacitor C₁ is discharged when power storage device 1 is not used (for example, when the ignition key is turned off).

FIG. 3 is a graph showing simulation results of operation states for respective constituents of power storage device 1 and discharge circuit 10. Upper part of FIG. 3 shows the voltage of capacitor C₁. Middle part of FIG. 3 shows the drain current I_(D) of transistor Q₁. Lower part of FIG. 3 shows power consumptions of transistor Q₁, resistor R₁, and resistor R₂.

In the present exemplary embodiment, capacitor C₁ is constituted by five electric double layer capacitors each having the maximum charging voltage of 2.5 V which are connected to one another. In a fully charged state, capacitor C₁ is charged to 12 V (refer to the upper part of FIG. 3).

At this time, if breakdown voltage V_(Z) of zener diode ZD₁ is set to 8.7 V; then a voltage more than or equal to breakdown voltage (8.7 V) is applied to zener diode ZD₁, and therefore, current I1 flows through zener diode ZD₁. Since resistors R8, R9 are connected in series to zener diode ZD₁, maximum current I_(1max) flowing through zener diode ZD₁ is limited to about several milliamperes to several ten milliamperes.

Moreover, a Hi signal of a predetermined voltage (for example, 2.5 V) more than or equal to V_(ref) is output from controller 20 to inverting input terminal (V−) of operational amplifier IC₁, and a zero voltage is output to the output terminal of operational amplifier IC₁. In this way, transistor Q₁ is turned off, and drain current I_(D) of transistor Q₁ is zero.

In this state, the discharge of capacitor C₁ is started. First, switch Q₄ is turned off, whereby the supply of power to capacitor C is stopped.

In this case, as mentioned above, large current I_(1max) is flowing through transistor Q₃ on the input side of current mirror circuit 15. Therefore, current I₂ (≈I_(1max)) of substantially the same magnitude also flows through transistor Q₂ on the output side of current mirror circuit 15. This current I₂ is supplied from reference power supply REF, and accordingly, potential V_(ER) of node ER of resistors R₅, R₆ becomes a value decreasing from reference voltage V_(ref) by a voltage drop due to resistor R₅ (V_(ER)=V_(ref)−I2·R5). Since current I₂ is large (≈I_(1max)), potential V_(ER) is reduced, and small voltage V_(ER) is input to non-inverting input terminal (V+) of operational amplifier IC₁.

Here, from controller 20, a Low signal of a predetermined voltage (for example, 0 V) less than or equal to V_(ref) is output to inverting input terminal (V−) of operational amplifier IC₁. In this way small voltage V_(ER) is output to the output terminal of operational amplifier IC₁, and the same voltage is also input to the gate of transistor Q₁. As a result, transistor Q₁ is turned on, drain current I_(D) starts to flow, and forced discharge of capacitor C₁ starts.

As when the discharge is started, when the voltage of capacitor C₁ is sufficiently larger than breakdown voltage V_(Z) of zener diode ZD₁ (“first section” in the upper part of FIG. 3), current I₁ flowing through transistor Q₃ is maintained at large current value I_(1max) due to the general feature of the zener diode mentioned above. Accordingly, current I₂ flowing through transistor Q₂ is also maintained at large current value I_(1max) in a similar way. In this way, potential V_(ER) of node ER and a potential of the output terminal of operational amplifier IC₁ are maintained at a predetermined low voltage. In this way, transistor Q₁ does not become a completely conductive state (full-on state), but as illustrated in the middle part of FIG. 3, drain current I_(D) of transistor Q₁ is maintained at a small value.

When the discharge of capacitor C₁ progresses, and the voltage of capacitor C₁ gradually decreases and becomes smaller than breakdown voltage V_(Z) of zener diode ZD₁ (“second section” in the upper part of FIG. 3), then current I₁ flowing through transistor Q₃ is reduced due to the general feature of the zener diode mentioned above. Following this reduction of current I₁, current I₂ flowing through transistor Q₂ is also reduced in a similar way, potential V_(ER) of node ER and the potential of the output terminal of operational amplifier IC₁ are gradually increased to approach voltage V_(ref) of the reference power supply. As a result, transistor Q₁ gradually approaches the full-on state, and as illustrated in the middle part of FIG. 3, drain current I_(D) of transistor Q₁ is also increased.

While capacitor C₁ is being discharged, controller 20 measures the voltage of capacitor C₁. When capacitor C₁ reaches a preset discharge ending voltage (for example, 5 V), controller 20 outputs the Hi signal to inverting input terminal (V−) of operational amplifier IC₁. In this way transistor Q₁ is turned off, and the discharge of capacitor C₁ is ended.

[Effects]

As mentioned above, in discharge circuit 10 according to the present exemplary embodiment, as illustrated in the middle part and the lower part of FIG. 3, such constant current discharge in which the discharge current (drain current I_(D)) is controlled to a small value is performed in the first section where the voltage of capacitor C₁ is relatively high. In this way power losses in transistor Q₁ and resistors R₁, R₂ can be prevented from being increased excessively. In the second section where the voltage of capacitor C₁ is reduced, drain current I_(D) is increased simultaneously with such a voltage drop of capacitor C₁. Accordingly, as illustrated in the lower part of FIG. 3, capacitor C₁ can be discharged such that the power losses in transistor Q₁ and resistors R₁, R₂ do not fluctuate largely. As described above, in the present exemplary embodiment, over the entire section of the discharge process of capacitor C₁, the power losses in transistor Q₁ and resistors R₁, R₂ do not fluctuate largely, and substantially constant discharge power is achieved.

That is, since capacitor C₁ can be discharged substantially evenly over the entire section of the discharge process, it is not necessary to use components specified adaptively to high power for transistor Q₁ and resistors R₁, R₂ which consume power of the discharge current. Hence, it is possible to reduce the size and cost of each of discharge circuit 10 and power storage device 1.

Moreover, in the present exemplary embodiment, zener diode ZD₁, resistors R₅ to R₇ and others are adjusted, whereby a discharge time of lowering the voltage of capacitor C₁ to a predetermined voltage can be controlled. For example, when breakdown voltage V_(Z) of zener diode ZD₁ is increased, the first section is shortened. That is, switching to the second section is made earlier. In this way, the discharge time of capacitor C₁ can be shortened. Moreover, when a resistance value of resistor R₅ is reduced, then a difference between reference voltage V_(ref) and voltage V_(ER) at node ER is reduced. Accordingly, input voltage V_(in) to non-inverting input terminal (V+) of operational amplifier IC₁ can be increased from the beginning of the discharge. In this way, drain current I_(D) of transistor Q₁ in the first section can be increased, and the entire discharge time can be shortened.

Moreover, while FIG. 3 illustrates an example where the voltage of 12 V fully charged to capacitor C₁ is lowered to 5 V within 20 minutes, the discharge time can be controlled within a range where a deterioration of cells composing capacitor C₁ (electric double layer capacitors in the present exemplary embodiment) does not progress.

Furthermore, in the present exemplary embodiment, the discharge of capacitor C₁ is ended when capacitor C₁ reaches the preset discharge ending voltage. In this way, predetermined electric charges remain in capacitor C₁ at the point of time when the discharge is ended. Accordingly a deterioration of the cells due to complete discharge can be prevented, and a charging time in the case where capacitor C₁ is charged next can be shortened. The voltage left in capacitor C₁ is a voltage at which capacitor C₁ does not deteriorate even if being left. The voltage is appropriately decided in accordance with a type and usage of capacitor C₁. In the present exemplary embodiment, the voltage of capacitor C₁ when the discharge is ended is set to 5 V. In this case, a residual voltage per cell is 1 V; and the deterioration of the electric double layer capacitor is considered hard to occur.

Moreover, in the present exemplary embodiment, controller 20 is capable of measuring source voltage ES of transistor Q₁. For example, transistor Q₁ is in off state when capacitor C is charged by battery BATT, and therefore, originally, no voltage is generated in the source of transistor Q₁. However, when transistor Q₁ is broken and so on, transistor Q₁ is turned on, and a voltage is generated in the source. Moreover, transistor Q₁ is in on state when capacitor C₁ is discharged, and therefore, originally, a voltage is generated in the source of transistor Q₁. However, when transistor Q₁ is broken and so on, transistor Q₁ is turned off, and no voltage is generated in the source. As described above, when the unintended voltage is detected in the source of transistor Q₁ in the case of charging or discharging capacitor C₁, transistor Q₁ is considered to be broken. Hence, abnormalities of discharge circuit 10 and power storage device 1 can be detected by measuring source voltage ES of transistor Q₁.

The discharge circuit and the power storage device have been described above based on the exemplary embodiment. However, the present disclosure is not limited to the above exemplary embodiment. For example, the scope of the present disclosure should include modifications which those skilled in the art can obtain by adding various design changes to the exemplary embodiment described above, as well as modifications implemented by freely combining constituents and functions described in the exemplary embodiment without deviating from the spirit of the present disclosure.

For example, capacitor C₁ provided in power storage device 1 is not limited to the electric double layer capacitor, and may be an electrolytic capacitor or a secondary battery. Capacitor C₁ may be a single cell or may have a configuration in which plural cells are combined with one another. For example, capacitor C₁ may have a configuration in which plural cells connected in series are connected in parallel or a configuration in which plural cells connected in parallel are connected in series.

Power storage device 1 includes battery BATT. However, the present disclosure is not limited to this, and the battery may be replaced by a generator. Moreover, power storage device 1 may be mounted not only on the vehicle but also on a home or industrial electrical instrument. Furthermore, battery BATT may be placed outside the power storage device. At this time, battery BATT may be a commercial alternating-current power supply (AC power supply).

Moreover, power storage device 1 includes controller 20, but the present disclosure is not limited to this, and the controller may be placed outside the power storage device. For example, it is also possible to control power storage device 1 using an electronic control unit (ECU) mounted on the vehicle.

Moreover, discharge circuit 10 includes reference power supply REF for supplying current I₂ to transistor Q₂, but the present disclosure is not limited to this, and it is also possible to supply a current from the outside of discharge circuit 10. 

What is claimed is:
 1. A method for controlling a discharge of a power storage device, the power storage device including a power storage and a discharge circuit, the method comprising: a first step of discharging, in a first section of a discharging period, a part of an electric charge stored in the power storage through a first discharge path and a second discharge path which are included in the discharge circuit; and a second step of discharging, in a second section of the discharging period, a remaining part of the electric charge stored in the power storage through the second discharge path, wherein: the first discharge path includes a zener diode connected to one end of the power storage, the second discharge path includes a first transistor connected to the one end of the power storage at a first input terminal of the first transistor, the first section is a period in which a voltage of the one end of the power storage is higher than a breakdown voltage of the zener diode, the second section is a period in which a voltage of the one end of the power storage is lower than the breakdown voltage of the zener diode, and an output current of the first transistor in the second section is greater than an output current of the first transistor in the first section.
 2. The method for controlling a discharge of a power storage device according to claim 1, wherein the output current of the first transistor in the second section increases as a voltage of the one end of the power storage decreases.
 3. The method for controlling a discharge of a power storage device according to claim 1, wherein a current flowing through the zener diode in the first section is greater than the output current of the first transistor in the first section.
 4. The method for controlling a discharge of a power storage device according to claim 2, wherein a current flowing through the zener diode in the first section is greater than the output current of the first transistor in the first section.
 5. A method for controlling a discharge of a power storage device, the power storage device including a power storage and a discharge circuit, the method comprising: a first step of discharging, in a first section of a discharging period, a part of an electric charge stored in the power storage through a first discharge path and a second discharge path which are included in the discharge circuit; and a second step of discharging, in a second section of the discharging period, a remaining part of the electric charge stored in the power storage through the second discharge path, wherein: the first discharge path includes a zener diode connected to one end of the power storage, the second discharge path includes a first transistor connected to the one end of the power storage at a first input terminal of the first transistor, the first section is a period in which a voltage of the one end of the power storage is higher than a breakdown voltage of the zener diode, the second section is a period in which a voltage of the one end of the power storage is lower than the breakdown voltage of the zener diode, and a current flowing through the zener diode in the first section is greater than the output current of the first transistor in the first section.
 6. The method for controlling a discharge of a power storage device according to claim 1, wherein: the power storage device further includes a controller, the controller includes an input terminal for detecting the voltage of the one end of the power storage and an output terminal controlling an operation of the first transistor, a discharge through the second discharge path is started by a discharge starting signal emitted from the output terminal of the controller, and is ended by a discharge ending signal emitted from the output terminal of the controller, the discharge starting signal is emitted in the first section, and the discharge ending signal is emitted in the second section.
 7. The method for controlling a discharge of a power storage device according to claim 2, wherein: the power storage device further includes a controller, the controller includes an input terminal for detecting the voltage of the one end of the power storage and an output terminal controlling an operation of the first transistor, a discharge through the second discharge path is started by a discharge starting signal emitted from the output terminal of the controller, and is ended by a discharge ending signal emitted from the output terminal of the controller, the discharge starting signal is emitted in the first section, and the discharge ending signal is emitted in the second section.
 8. The method for controlling a discharge of a power storage device according to claim 3, wherein: the power storage device further includes a controller, the controller includes an input terminal for detecting the voltage of the one end of the power storage and an output terminal controlling an operation of the first transistor, a discharge through the second discharge path is started by a discharge starting signal emitted from the output terminal of the controller, and is ended by a discharge ending signal emitted from the output terminal of the controller, the discharge starting signal is emitted in the first section, and the discharge ending signal is emitted in the second section.
 9. The method for controlling a discharge of a power storage device according to claim 4, wherein: the power storage device further includes a controller, the controller includes an input terminal for detecting the voltage of the one end of the power storage and an output terminal controlling an operation of the first transistor, a discharge through the second discharge path is started by a discharge starting signal emitted from the output terminal of the controller, and is ended by a discharge ending signal emitted from the output terminal of the controller, the discharge starting signal is emitted in the first section, and the discharge ending signal is emitted in the second section.
 10. The method for controlling a discharge of a power storage device according to claim 5, wherein: the power storage device further includes a controller, the controller includes an input terminal for detecting the voltage of the one end of the power storage and an output terminal controlling an operation of the first transistor, a discharge through the second discharge path is started by a discharge starting signal emitted from the output terminal of the controller, and is ended by a discharge ending signal emitted from the output terminal of the controller, the discharge starting signal is emitted in the first section, and the discharge ending signal is emitted in the second section.
 11. The method for controlling a discharge of a power storage device according to claim 1, wherein: the power storage device further includes a controller, the controller includes an input terminal for detecting the voltage of the one end of the power storage and an output terminal controlling an operation of the first transistor, the discharge circuit further includes an operational amplifier and a current mirror circuit, the operational amplifier controls the output current of the first transistor and is connected to a second input terminal of the first transistor, an output terminal of the current mirror circuit is connected to the operational amplifier, an input terminal of the current mirror circuit is connected to the one end of the power storage via the zener diode, the current mirror circuit includes a second transistor and a third transistor, an output terminal of the second transistor is connected to a first input terminal of the operational amplifier, and an input terminal of the third transistor is connected to the one end of the power storage via the zener diode.
 12. The method for controlling a discharge of a power storage device according to claim 2, wherein: the power storage device further includes a controller, the controller includes an input terminal for detecting the voltage of the one end of the power storage and an output terminal controlling an operation of the first transistor, the discharge circuit further includes an operational amplifier and a current mirror circuit, the operational amplifier controls the output current of the first transistor and is connected to a second input terminal of the first transistor, an output terminal of the current mirror circuit is connected to the operational amplifier, an input terminal of the current mirror circuit is connected to the one end of the power storage via the zener diode, the current mirror circuit includes a second transistor and a third transistor, an output terminal of the second transistor is connected to a first input terminal of the operational amplifier, and an input terminal of the third transistor is connected to the one end of the power storage via the zener diode.
 13. The method for controlling a discharge of a power storage device according to claim 3, wherein: the power storage device further includes a controller, the controller includes an input terminal for detecting the voltage of the one end of the power storage and an output terminal controlling an operation of the first transistor, the discharge circuit further includes an operational amplifier and a current mirror circuit, the operational amplifier controls the output current of the first transistor and is connected to a second input terminal of the first transistor, an output terminal of the current mirror circuit is connected to the operational amplifier, an input terminal of the current mirror circuit is connected to the one end of the power storage via the zener diode, the current mirror circuit includes a second transistor and a third transistor, an output terminal of the second transistor is connected to a first input terminal of the operational amplifier, and an input terminal of the third transistor is connected to the one end of the power storage via the zener diode.
 14. The method for controlling a discharge of a power storage device according to claim 4, wherein: the power storage device further includes a controller, the controller includes an input terminal for detecting the voltage of the one end of the power storage and an output terminal controlling an operation of the first transistor, the discharge circuit further includes an operational amplifier and a current mirror circuit, the operational amplifier controls the output current of the first transistor and is connected to a second input terminal of the first transistor, an output terminal of the current mirror circuit is connected to the operational amplifier, an input terminal of the current mirror circuit is connected to the one end of the power storage via the zener diode, the current mirror circuit includes a second transistor and a third transistor, an output terminal of the second transistor is connected to a first input terminal of the operational amplifier, and an input terminal of the third transistor is connected to the one end of the power storage via the zener diode.
 15. The method for controlling a discharge of a power storage device according to claim 5, wherein: the power storage device further includes a controller, the controller includes an input terminal for detecting the voltage of the one end of the power storage and an output terminal controlling an operation of the first transistor, the discharge circuit further includes an operational amplifier and a current mirror circuit, the operational amplifier controls the output current of the first transistor and is connected to a second input terminal of the first transistor, an output terminal of the current mirror circuit is connected to the operational amplifier, an input terminal of the current mirror circuit is connected to the one end of the power storage via the zener diode, the current mirror circuit includes a second transistor and a third transistor, an output terminal of the second transistor is connected to a first input terminal of the operational amplifier, and an input terminal of the third transistor is connected to the one end of the power storage via the zener diode.
 16. The method for controlling a discharge of a power storage device according to claim 6, wherein: the power storage device further includes a controller, the controller includes an input terminal for detecting the voltage of the one end of the power storage and an output terminal controlling an operation of the first transistor, the discharge circuit further includes an operational amplifier and a current mirror circuit, the operational amplifier controls the output current of the first transistor and is connected to a second input terminal of the first transistor, an output terminal of the current mirror circuit is connected to the operational amplifier, an input terminal of the current mirror circuit is connected to the one end of the power storage via the zener diode, the current mirror circuit includes a second transistor and a third transistor, an output terminal of the second transistor is connected to a first input terminal of the operational amplifier, and an input terminal of the third transistor is connected to the one end of the power storage via the zener diode.
 17. The method for controlling a discharge of a power storage device according to claim 7, wherein: the power storage device further includes a controller, the controller includes an input terminal for detecting the voltage of the one end of the power storage and an output terminal controlling an operation of the first transistor, the discharge circuit further includes an operational amplifier and a current mirror circuit, the operational amplifier controls the output current of the first transistor and is connected to a second input terminal of the first transistor, an output terminal of the current mirror circuit is connected to the operational amplifier, an input terminal of the current mirror circuit is connected to the one end of the power storage via the zener diode, the current mirror circuit includes a second transistor and a third transistor, an output terminal of the second transistor is connected to a first input terminal of the operational amplifier, and an input terminal of the third transistor is connected to the one end of the power storage via the zener diode.
 18. The method for controlling a discharge of a power storage device according to claim 8, wherein: the power storage device further includes a controller, the controller includes an input terminal for detecting the voltage of the one end of the power storage and an output terminal controlling an operation of the first transistor, the discharge circuit further includes an operational amplifier and a current mirror circuit, the operational amplifier controls the output current of the first transistor and is connected to a second input terminal of the first transistor, an output terminal of the current mirror circuit is connected to the operational amplifier, an input terminal of the current mirror circuit is connected to the one end of the power storage via the zener diode, the current mirror circuit includes a second transistor and a third transistor, an output terminal of the second transistor is connected to a first input terminal of the operational amplifier, and an input terminal of the third transistor is connected to the one end of the power storage via the zener diode.
 19. The method for controlling a discharge of a power storage device according to claim 9, wherein: the power storage device further includes a controller, the controller includes an input terminal for detecting the voltage of the one end of the power storage and an output terminal controlling an operation of the first transistor, the discharge circuit further includes an operational amplifier and a current mirror circuit, the operational amplifier controls the output current of the first transistor and is connected to a second input terminal of the first transistor, an output terminal of the current mirror circuit is connected to the operational amplifier, an input terminal of the current mirror circuit is connected to the one end of the power storage via the zener diode, the current mirror circuit includes a second transistor and a third transistor, an output terminal of the second transistor is connected to a first input terminal of the operational amplifier, and an input terminal of the third transistor is connected to the one end of the power storage via the zener diode.
 20. The method for controlling a discharge of a power storage device according to claim 10, wherein: the power storage device further includes a controller, the controller includes an input terminal for detecting the voltage of the one end of the power storage and an output terminal controlling an operation of the first transistor, the discharge circuit further includes an operational amplifier and a current mirror circuit, the operational amplifier controls the output current of the first transistor and is connected to a second input terminal of the first transistor, an output terminal of the current mirror circuit is connected to the operational amplifier, an input terminal of the current mirror circuit is connected to the one end of the power storage via the zener diode, the current mirror circuit includes a second transistor and a third transistor, an output terminal of the second transistor is connected to a first input terminal of the operational amplifier, and an input terminal of the third transistor is connected to the one end of the power storage via the zener diode. 